Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by a layer of fluid. Sometimes, multiple layers of a coating are applied on top of each other. After the deposition of a coating, it can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure-sensitive adhesive will not aggressively stick. Methods of applying coating are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vorstrand Reinhold Publishing Co., New York 1984. It is desired and necessary in many situations to coat ultra-thin layers which are no more than 5 microns thick.
Of the coating methods known for applying continuous fluid coatings (such as roll, curtain, slot, air knife, slide, and gravure coating) other than water expansion techniques, none can apply wet coating thicknesses below about 0.1 micron. To achieve lower final dry thicknesses with these methods, the coating must be diluted with a solvent that can be removed by evaporation to leave behind the desired coating below about 0.1 micron. This increases costs by adding the cost of the diluent, the cost of preparing the diluted coating fluid, and the cost of removing the diluent (such as by drying). Also, the necessary solvent is often hazardous to the environment and the manufacturing personnel.
Discontinuous methods of applying ultra-thin coatings molecule by molecule or drop by drop include condensing from a vapor phase and the electrospray process described in U.S. Pat. No. 4,748,043. However, few fluid coatings of commercial interest can be successfully vaporized, and the electrospray process is limited to a narrow range of viscosity and electrical properties of the coating fluid.
For thicknesses greater than 0.1 micron, multiple roll or transfer roll coaters are used. Typical commercial equipment includes the five roll coater sold by the Bachofen & Meier AG, of Bulach, Germany, and others. This coater style is expensive to purchase and maintain because of its many driven rolls. Any defect in the surface of the rolls usually produces a repeating defect in the coating. Additionally, these coaters have not successfully applied wet coatings in the 0.005 to 0.1 micron range.
Water expansion techniques first started with the Langmuir-Blodgett method of producing and depositing monomolecular films as described by Blodgett in the Journal of the American Chemical Society (Vol. 57, 1007, 1935). This method involves casting a dilute solvent solution of a film-forming organic molecule on a stagnant water surface. The solution spreads to form a thin film on the water-air interface. The solvent is evaporated leaving behind a monolayer of film forming molecules. The film is then deposited on the surface of a substrate by passing the substrate through the water surface on which the monomolecular layer film is riding. U.S. Pat. No. 4,093,757 discloses forming a continuous monomolecular deposit on a continuous substrate. Japanese Patent Application 63-327260 discloses an improvement of the Langmuir-Blodgett monomolecular technology where films greater than a monomolecular layer thick are deposited on a continuous substrate to form an ultra-thin film coating at thicknesses of 0.005 to 5 microns.
Although the water surface expansion technique can coat useful coatings on substrates, it requires that the coating fluid spontaneously and rapidly spread on the water-air interface. To achieve this for many coating formulations, additional solvents or surface active agents must be found and added. Additionally, the maximum coating rate is limited by the rate of spreading. Also, the speed of coating the substrate is limited by other problems. It is reported that at modest speeds of 10 to 50 m/minute, air bubbles tend to be trapped between the film and the substrate. Water expansion techniques rely on picking the coating from a stagnant water pond by passing the substrate through the water surface or contacting the substrate to the water surface. Often, evaporation of solvent must occur to create a solid or near solid surface film to allow the direct transfer of the coating to the substrate. The surface of the pond is also subject to contamination that can increase with time, degrading the coating quality. The water expansion technique is not known to be useable with miscible coating fluids and water soluble or dispersible coating constituents.